This is legacy content as it has been replaced by use of the Elo rating system.
It is the summation of the Gladiatorship ratings of all the members of a guild.
This was discovered by looking at the guilds at the bottom of the pantheon of aggressiveness and finding guilds whose members all have 0/0 duel records apart from just one of them, based on the assumption that only the duel record affects this rating (from the pantheon description). This ensured that only that one duel record was responsible for the rating.
After a similar process to figuring out the gladiatorship rating formula (imagining what the formula should be like), a formula was deduced that was identical to the gladiatorship rating formula. A quick testing for all of these sets of values revealed this to be the correct formula. This was followed by a large facepalm and a subsequent cup of coffee.